In excimer lasers, as is well known, a lasing medium is generated by highly excited excimer molecules being formed in the plasma of a gas discharge to which noble gas atoms and e.g. halogens or hydrogen halides (plus other reactants) contribute.
The lifetime of excimer laser gases is limited mainly by two processes. On the one hand, obscure chemical reactions between the halogen and the container do take place even in well passivated systems, entailing consumption of the halogen and a rise of the contamination level. On the other hand, chemical reactions are caused also by the minor electrode burn-off which takes place during high voltage discharge. The loss in laser performance in the course of time is compensated--normally by user-friendly and efficient computer control systems--by raising the charging voltage of the laser and/or regenerating the laser gas or replacing it in part or totally.
The operating pressure of an excimer laser typically is from 2.8 to 3.8 bars, absolute. While supplying halogen by means of pressure cylinders (usually containing F.sub.2 and HCl gases diluted with helium) permits operating at high pressures in filling or refilling the laser gas reservoir, a more recently suggested way of supplying the gas by producing the desired halogen in situ (see e.g. German patent application P 40 36 963.3) provides the gas approximately at room temperature and at pressures of from a few millibars to a few 100 millibars at volumes which can be handled, ranging from about 100 to 500 cm.sup.3.
These pressures are sufficient for charging a laser reservoir (typically containing from about 30 to 60 liters), a procedure which starts from an absolute pressure of about 20 mbar, provided the halogen portion (e.g. F.sub.2 or HCl) is filled in first and then the noble or buffer gas share is added until the final operating pressure is reached. Once it is reached, the laser usually is started and continues to run for several millions of impulses, with or without cleansing of the gas, until the halogen concentration has become so reduced that control of the laser output performance level can be achieved only by halogen injection. In other words, the gas reservoir of the laser must be recharged with halogen (such as F.sub.2 or HCl) for the performance to be maintained. This halogen injection which either may be only F.sub.2 or only HCl, depending on the operating gas, normally accounts for no more than a fraction (approximately 1/10 to 1/4) of the halogen partial pressure required for a complete gas exchange, which typically is 5 mbar in the gas reservoir of the laser.
With the laser in operation, this quantity cannot be refilled without much compression of the halogen if the halogen, for instance, comes from a gas generator providing the same in situ.